Materials Science Forum Vols. 783-786

Abstract: In the present study fretting fatigue behaviour of 12-Cr steels at 300°C has been investigated under three different contact pressures. For comparisons fretting fatigue behaviour of 12-Cr steels at room temperature has also been investigated. The result showed that with an increase in contact pressure and temperature, the fretting fatigue significantly reduces. Finite element analyses were carried out to evaluate the stress distribution (tangential stress and compressive stress) at the contact during fretting fatigue. Tangential stress range – compressive stress range diagram (TSR-CSR diagram) were constructed for 12-Cr steel at room temperature and at 300°C. Then, a generalized TSR-CSR diagram to predict fretting fatigue strength of 12-Cr steel regardless of contact pressure and temperature was constructed.

Abstract: Aggregate of bainite laths with almost parallel slip systems between neighboring bainite laths, hereafter referred to as ALPS, has great effect on the improvement of mechanical properties in steels. Elongation increases remarkably with increasing the number of bainite laths within an ALPS. When a bainite lath begins to deform, the neighboring bainite lath also easily deforms to relax the deformation strain, because of good parallelism between their slip systems. Under the cooperative deformation of bainite laths, the area of interface between neighboring bainite laths increases during deformation. The increase in the area of interface between neighboring bainite laths suppresses localization of dislocations at the interface, that is, dislocation density per unit area of the interface between neighboring bainite laths hardly increases, resulting in the suppression of nucleation of cracks at the interface between neighboring bainite laths. Ductile fracture would occur along the boundary between ALPSs. It could be suggested that larger ALPS and/or ALPS consisting of large number of bainite laths induce larger elongation in steels.On the other hand, it has been reported that tensile strength increases in proportion to inverse of square root of d, the d being the average size of bainite laths [1]. In order to form fine bainite lath, dislocation network instead of inclusion in austenite was utilized as nucleation site for bainite lath. Great barriers to be overcome exist for the improvement of both strength and toughness. An idea for the improvement of both strength and toughness is shown in this study.

Abstract: Effect of pre-treatments before austenitization on mechanical properties in steels has been investigated. Pre-treatments are cold rolling and isothermal holding below A₁ before austenitization. Pre-treatments were performed in order to introduce dislocation network in austenite (γ), which acts as nucleation site for “bainite laths within γ grain (BWING)”. During the pre-treatment of isothermal holding below A₁, carbon segregation and/or carbide formation might occur at grain boundary of ferrite (α). Thin γ layer might be formed around the region where carbon segregation and/or carbide formation occurred in the beginning of austenitization. Transformation strain due to α → γ would be effectively relaxed by the deformation of thin γ film, because α had been already hardened by the cold rolling. Dislocations might be introduced dominantly into the thin γ film, and stable dislocation network even in the high temperature region of γ might be formed. BWING nucleates around the dislocation network to relax the strain field around dislocation network. BWING would nucleate also at ( BWING / γ ) interface cooperatively to relax the strain around the interface, resulting in the formation of “aggregate of bainite laths with nearly parallel slip systems between neighboring bainite laths (ALPS)”. Larger ALPS containing a lot of BWINGs might be formed in the pre-treated steel, and improves tensile strength and toughness.

Abstract: Analysis of standard and experimental lean alloyed chemical composition for pipeline steel X80 was made. Rolling schedules for experimental X80 chemical composition by means of computer simulation using HSMM, AusEvol+ and AusTran software were developed for continuous mill 2000. Developed schedules were analyzed in order to choose one of them which guarantees the required microstructure receiving. The chosen schedule was realized on Gleeble-3800 system by tension-compression tests for experimental chemical composition. Microstructure analysis and mechanical testing of received samples was made to compare results with requirements submitted to pipeline steel X80.

Abstract: Superior fatigue life of 8000 cycles at low-cycle fatigue with a total strain Δε=2% was found in the Fe–30Mn–4Si–2Al high-Mn alloy, as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys with fatigue life of 2×10³ cycles. Examination of microstructural evolution and cyclic hardening/softening behavior was shown that high fatigue resistance of Fe–30Mn–4Si–2Al alloy associated with delayed development of the deformation induced martensite and inhibited dislocation slip as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys, respectively. Cyclic strain softening followed by secondary strain hardening was observed in the Fe–30Mn–4Si–2Al alloy after primary hardening. Primary hardening to about 40 cycles was associated with continuous increase in density of planar dislocations and the development of slip bands. The cyclic softening manifesting as the drop of the stress amplitude in the range of the cycles from 40 to 400 was accompanied by development of deformation induced ε-martensite in place of the slip bands. At the N>400 cycles further increase in the volume fraction of deformation ε-martensite leads to continuous hardening up to the failure. In the presentation we will discuss the details of microstructural evolution during LCF of the Fe–30Mn–4Si–2Al alloy.

Abstract: This study aims to develop novel experimental procedure that quantifies response of AHSSs with different microstructures, deformation status, and strength levels to hydrogen. The capacity for trapped hydrogen, kinetics of hydrogen absorption and loss, and hydrogen mobility are measured and analyzed by permeation tests. The experimental findings are discussed in terms of microstructural features for an interstitial free (IF) and a dual phase (DP) steels. Further, the density of trap sites and its effect on effective diffusivity of hydrogen in the steel are analyzed by means of a diffusion model.

Abstract: Blast Furnace remains to be one of main producers of molten iron. The secondary or alternate sources of producing molten iron have come in place and contribute around 30% of molten iron production in the world. Good iron ore reserves are fast depleting coupled with a huge amount of fines being produced during mining.

Abstract: The effect of phases and steel processing on hydrogen uptake (diffusible and residual), surface and internal damage were evaluated using optical and scanning electron microscopy. The results have shown the fastest formation of blisters in ferrite-pearlite microstructure of strip, followed by equaixed ferrite-pearlite microstructure in normalised condition, then by ferrite-bainite microstructure. No blistering was observed in heat affected zone samples for up to 24 hrs charging. Analysis of hydrogen-induced cracking using electron back scattering diffraction has revealed that crack propagation has predominantly intragranular character without a clear preference on {001}, {110}, {112} and {123} planes and is independent of the steel microstructure and prior processing.

Abstract: Hot strip rolling of steels inherently results in non-homogeneous microstructure and mechanical properties of hot bands. Thermomechanical processing that implies careful selection of rolling temperatures, speeds, reductions and controlled cooling parameters, as well as their accurate in-bar dynamic control allows not only for reducing the inherent microstructure variability but also for attenuating and even eliminating the adverse downstream manifestations of microstructure non-homogeneities. This is especially pertinent to advance high strength steels (AHSS) for automotive applications that have been shown to possess high sensitivity to variations in industrial processing conditions. Examples of industrial data and real time monitoring of hot band microstructure evolution using online non-destructive technique are presented confirming the efficiency of thermomechanical processing in ensuring the proper quality of AHSS sheet products.

Abstract: Superduplex steels exhibit a microstructure of approximately equivalent fractions of austenite and δ-ferrite. This structure combines a higher strength than austenitic steels with a higher toughness than ferritic steels and an excellent corrosion resistance. Superduplex steels can be processed by different routes such as casting, extrusion, rolling or forging and are applied in the chemical industry, oil production or paper manufacturing. It is well known that the two phases exhibit a different dynamic restoration behavior due to their differences in the stacking fault energy. The austenite grains are more likely to undergo discontinuous dynamic recrystallization while the δ-ferrite grains tend to strong dynamic recovery. Modern large area electron back scatter diffraction (EBSD) scans are a powerful technique to study the microstructural evolution of the individual phases during hot-forming of duplex steels. However, detailed EBSD studies explaining the flow behavior, influence of grain orientation and grain size modification during hot forming have not been carried out yet. In the present investigation specimens of a S32750 superduplex steel were deformed in uniaxial compression with strain rates between 0.01 and 1 [s^-1] to a true strain of 1 at temperatures between 1000 and 1300°C. The microstructures of the as-deformed specimens were examined by large area EBSD scans with particular attention to the characteristics of the individual phases as for example grain size and stored energy of the austenite grains or subgrain size and grain boundary character of the ferrite grains. The differences due to deformation temperature and strain rate are discussed.